Dissociation of modular humeral head components: a biomechanical and implant retrieval study

A novel method to improve femoral head and stem taper stability intraoperatively in total hip arthroplasty - a proof of concept study

BACKGROUND

Mechanically assisted crevice corrosion (MACC) has been associated with the compromised durability and fixation of modular total hip implants, adverse reaction of local tissue, and other undesirable clinical outcomes in total hip arthroplasty (THA). MACC is primarily caused by the relative motion between the femoral head and stem. To minimize the relative motion the taper connection between the two components must be strong enough. The current study addressed the following questions: (1) Does increasing the mass of the femoral stem improve the taper connection strength intraoperatively? (2) Does increasing the mass of the femoral stem reduce the risk of periprosthetic tissue damage intraoperatively?

HYPOTHESIS

Increasing the mass of the femoral stem improve the taper connection strength intraoperatively.

MATERIALS AND METHODS

During the experiment, femoral heads were impacted onto the stem tapers with and without an additional weight attached to the stem. The femoral heads were then pulled off to investigate the strength of the taper connection. The stem displacement and acceleration at impaction were also measured to evaluate the risk of periprosthetic tissue damage.

RESULTS

The results showed that the pull-off force was increased by 24% (p=0.011, n=6) when an additional weight was attached to the stem. The additional weight also reduced the maximum stem acceleration and maximum stem displacement by 37% (p<0.001, n=6) and 14% (p=0.094, n=6), respectively.

DISCUSSION

These findings suggest that the femoral head and stem taper connection strength can be significantly improved and the risk of periprosthetic tissue damage significantly reduced intraoperatively by attaching an additional weight to the stem to increase its mass.

LEVEL OF EVIDENCE

III, comparative in vitro mechanical investigation.

Biomechanical Evaluation of Recurrent Dissociation of Modular Humeral Prostheses

The purpose of the study was to evaluate the force and torque required to dissociate a humeral head from the unimplanted modular total shoulder replacement system from different manufacturers and to determine if load and torque to dissociation are reduced in the presence of bodily fluids. Impingement, taper contamination, lack of compressive forces, and interference of taper fixation by the proximal humerus have all been reported as possible causes for dissociation. Experimental values determined in this research were compared with literature estimates of dissociation force of the humeral head under various conditions to gain more understanding of the causes of recurrent dissociations of the humeral head. This study examined biomechanical properties under dry and wet conditions under clinically practiced methods. Mean load to dissociation (1513 N ± 508 N) was found to be greater than that exerted by the activities of daily living (578 N) for all implants studied. The mean torque to dissociation was (49.77 N·m ± 19.07 N·m). Analysis of R² correlation coefficients and p-values (α = 0.05) did not show any significant correlation between dry/bovine, dry/wet, or wet/bovine for load, displacement, or torsional stiffness in the majority of tests performed. Wetting the taper with water or bovine serum did not reduce the dissociation force to a statistically significant degree. Torque and lack of compressive forces at the rotator cuff may be the cause of dissociation at values less than those of activities of daily living. Torque data are provided by this study, but further research is needed to fully appreciate the role of torque in recurrent dissociations.

Taper junctions in modular hip joint replacements: What affects their stability?

BACKGROUND

Although taper junctions are beneficial in the reconstruction of hip joints, some clinical concerns like the formation of adverse local tissue reactions have recently emerged. These reactions are associated with wear and corrosion products from the interface of insufficient taper connections regarding strength. Commonly used tapers vary in their geometric and topographical design parameter. Therefore, this study aims to evaluate interactions between design and surgical related parameters to the taper connection strength.

METHODS

In this study, the effect of the taper contact situation, surface roughness and head material in combination with assembly force on the taper connection strength were assessed using torque-off tests. Furthermore, the type of use in terms of single-use or re-use of the stem taper was investigated.

RESULTS

The study showed that the impaction force is the predominant factor that determines the taper strength followed by the type of use and the head material. The contact situation seems to slightly influence the determined torque-off moment, whereas the surface topography of the stem taper obviously plays a minor role for the taper connection strength.

CONCLUSION

Clinical users should be aware that an increased assembly force will strengthen the stability of the taper junction, whereas care should be taken when reusing hip stems with metal heads as this may decrease their connection strength.

Failure at the femoral stem extension-condylar interface in a rotating hinge knee without radiographic evidence of loosening

Modular stem extensions have become ubiquitous in revision total knee arthroplasty systems. Although stem extensions are valuable in addressing bone deficiencies and improving implant fixation, the stem extension-condylar interface may be a point of implant failure. We report a case of failure at the femoral stem extension-condylar interface in a Zimmer NexGen Rotating Hinge Knee (Zimmer, Warsaw, IN). Currently, several published case reports describe failure at the femoral stem extension-condylar interface but only 1 case describes loosening at a taper junction without evidence of set screw failure or taper fracture. Furthermore, no published cases describe this type of failure in the Zimmer NexGen Rotating Hinge Knee (Zimmer).

Late Partial Dissociation of a Humeral Head Morse Taper Associated with Periprosthetic Proximal Humeral Fracture: A Case Report

CASE

A fifty-two-year-old man who had undergone total shoulder arthroplasty ten years previously presented following a mechanical fall onto his operatively treated shoulder, resulting in a periprosthetic humeral fracture and partial dissociation of the humeral head from the stem.

CONCLUSION

Shoulder surgeons should consider late disengagement of the Morse taper as a potential cause of surgical failure. Partial late dissociation of the humeral head from the stem after shoulder arthroplasty is possible and can be associated with a fracture. The shoulder surgeon should be aware of this potential complication when evaluating periprosthetic proximal humeral fractures, emphasizing the importance of attention to detail when evaluating patients.

Glenosphere dissociation after reverse shoulder arthroplasty

BACKGROUND

Reverse shoulder arthroplasty (RSA) is gaining popularity for the treatment of debilitating shoulder disorders. Despite marked improvements in patient satisfaction and function, the RSA complication rate is high. Glenosphere dissociation has been reported and may result from multiple mechanisms. However, few RSA retrieval studies exist.

METHODS

We reviewed our RSA database and identified patients with glenosphere dissociation between 1999 and 2013. Prosthesis type, glenosphere size, and contributing factors to dissociation were noted. Five retrieved implants were available for analysis, and evidence of wear or corrosion on the Morse taper was documented. Further, we biomechanically investigated improper Morse taper engagement that may occur intraoperatively as a potential cause of acute dissociation.

RESULTS

Thirteen patients with glenosphere dissociation were identified (0.5 months to 7 years postoperatively). Glenosphere size distribution was as follows: 32 mm (n = 1), 36 mm (n = 4), 40 mm (n = 6), and 44 mm (n = 2). Incidence of dissociation was correlated to glenosphere size (P < .001). Taper damage was limited to fretting wear, and there was minimal evidence of taper corrosion. Biomechanically, improper taper engagement reduced the torsional capacity of the glenosphere-baseplate interface by 60% from 19.2 ± 1.0 N-m to 7.5 ± 1.5 N-m.

CONCLUSION

We identified several mechanisms contributing to glenosphere dissociation after RSA, including trauma and improper taper engagement. Limited evidence of corrosive wear on the taper interface was identified. Although it is rare, the incidence of glenosphere dissociation was higher when 40- and 44-mm glenospheres were implanted compared with smaller glenospheres (32 and 36 mm), probably because of the larger exposed surface area for potential impingement.

Influence of assembly procedure and material combination on the strength of the taper connection at the head-neck junction of modular hip endoprostheses

BACKGROUND

A stable fixation between femoral head and endoprosthesis taper is necessary to prevent relative motions and corrosion at the taper junction. Although the importance of the component assembly has been recognised, no definitive instructions are available. The purpose of this study was to assess the influence of assembly force, assembly tool and number of hammer strokes on the taper junction strength of various material combinations.

METHODS

Co-Cr29-Mo (n=10) and Ti-6Al-4V (n=10) neck tapers were assembled with Co-Cr and Al(2)O(3) ceramic heads either by push-on or by impaction with single or multiple hammer blows. The strength of the taper-head connection was evaluated by measuring the head pull-off forces according to ISO 7206-10 and the turn-off moment capacity.

FINDINGS

The taper strength linearly increased with assembly forces (P<0.001). Co-Cr heads combined with Co-Cr tapers showed significantly lower pull-off forces and turn-off moments than the combination with Ti tapers (0.001<P<0.025). Multiple impaction did not increase taper strength (0.063 <P<0.995). Ceramic and Co-Cr heads showed similar fixation patterns on Ti tapers. Turn-off moments varied between 6 Nm and 19 Nm, dependent on material combination and assembly force.

INTERPRETATION

It is suggested that sufficient head-taper junction strength in all bearing conditions is achieved by impaction forces of at least 4 kN. A single impact is sufficient to achieve fixation. Special attention should be paid to the assembly of Co-Cr heads on Co-Cr tapers.

Hybrid assembly of metal head and femoral stem from different manufacturers during isolated acetabular revision

During revision of a failed acetabular component with well-fixed antiquated femoral stem, the same variant of metal head is usually not available in the inventory production. We gathered the stems retrieved from revision surgeries and tried to reassemble with the commercially available femoral heads. This study reports the results of suitable hybrid assembly of metal heads with the most common antiquated femoral stems from different manufacturers. Three hybrid combinations were determined and pull-off strengths of these hybrid combinations were performed through in vitro studies to evaluate the quality of the taper locks.

Modular ulnar head decoupling force: case report

Cobalt-chrome modular distal ulnar head replacement arthroplasty is a surgical option to restore stability to the distal radioulnar joint rendered unstable by hemi-resection arthroplasty or a total resection arthroplasty. However, the revision of dislocated modular cobalt-chrome ulnar head implants may pose an important intraoperative challenge. The Morse-taper disassembly force of modular ulnar head implants is not available in the current published literature. We present a case in which tremendous difficulty was encountered while revising a dislocated modular cobalt-chrome distal ulnar head implant. The mean Morse-taper disassembly force of the retrieved modular cobalt-chrome implant was 2958 N +/- 1272. At nearly 4.5 times the average body weight, the modular ulnar head Morse-taper disassembly strength presented a formidable force to overcome intraoperatively.

A case of disassociation of a modular femoral neck trunion after total hip arthroplasty

Modular femoral heads offer the advantages of increased intraoperative flexibility through the adjustment of leg length and offset, whereas a modular femoral neck can also allow independent adjustment of femoral anteversion. Despite the potential advantages of hip systems using increased modularity, these component designs also offer a greater number of junctions through which problems may occur. This case demonstrates the potential for dissociation of a Morse taper between a modular femoral neck and stem.

Effect of impact assembly on the fretting corrosion of modular hip tapers

The goal of this study was to determine the effect of assembly load and local assembly environmental conditions on the fretting corrosion of modular femoral stem tapers. Femoral head/taper assemblies in both similar (CoCrMo/CoCrMo) and mixed (CoCrMo/Ti-6Al-4V) alloy combinations were evaluated using an electrochemical test method. Specimens were assembled under impact loading and by hand, in both wet and dry conditions. Incremental cyclic loads ranging from 89 to 5,340 N were applied at a frequency of 3 Hz in Ringer's solution at ambient temperature. During the test, both the open circuit potential (OCP) and fretting current (i(fret)) were measured using a saturated calomel electrode (SCE) and counter electrode, respectively. The results were comparable for both mixed and similar alloy couples. Decreases in OCP and increases in i(fret) (indicators of oxide film fracture and repassivation) were seen with increasing load magnitude, often occurring at loads well below those expected clinically. OCP at the 5,340 N cyclic load ranged from -30.4 to -103.7 mV versus SCE for similar alloy couples, and -19.1 to -181.4 mV versus SCE for mixed alloy couples. Mean peak fretting currents ranged from 0.84 to 1.42 microA and 1.06 to 3.12 microA for similar and mixed alloy couples, respectively. The larger current magnitudes and more negative shifts in OCP for mixed alloy couples indicate the difference in oxide film fracture behavior between titanium and cobalt alloys. The load at which OCP began to drop (onset of fretting) was dependent upon the assembly conditions for both material couples. Specimens assembled with impact loads in air showed the highest resistance to fretting. The results of this study indicate that the assembly load and the environment both play a role in the initial stability of modular hip taper connections.

Complications with humeral head replacement

Humeral head replacement has been shown to be an excellent surgical option for a variety of debilitating shoulder conditions. New techniques and advances in prosthetic design have furthered the success of this procedure. When performed without appropriate indications, exacting technique, and a tailored rehabilitation program, however, complications often result. Early recognition and appropriate management of these complications aid in achieving a satisfactory result.